This invention relates to an x-ray detector for use in automatic exposure control in x-ray equipment, and in particular to an x-ray detector for use in scanning beam x-ray radiographic equipment.
Control of the exposure of x-ray film, or of other x-ray sensitive media, is necessary to obtain the maximum diagnostic information from the recorded x-ray image. The limited exposure range of most such media causes a loss of image detail, as conveyed in the contrast of the image, if the media is underexposed or overexposed. Overexposure of the media will reduce the contrast of imaged body structures that are relatively transparent to x-rays. Underexposure will reduce the contrast of imaged body structures that are relatively opaque to x-rays
Accurate exposure is particularly important in the imaging of soft tissue, as in applications such as mammography, where the differences of x-ray absorption between different tissue is low and where the thickness of the tissue and therefore the amount of x-rays transmitted by the tissue varies substantially over the image area.
Recording several images at different x-ray exposures is often required to obtain the correct exposure. The drawback to this approach is that it requires that the patient be exposed to additional x-ray radiation and it requires additional time and expense. Alternatively, the contrast of the image may be reduced by adjusting the KVP of the x-ray tube so as to allow more exposure latitude. This approach, however, reduces the ability of the diagnostician to detect low contrast objects.
In a conventional "area beam" x-ray apparatus, the exposure may be controlled by changing the exposure time. The exposure over the entire image area is uniform and therefore automatic exposure control is possible with the use of small area ionization-type or semiconductor x-ray detectors. Such detectors are centered within the image area to read the x-ray exposure within the detector's area to control the exposure of the entire area beam.
More recent, scanning x-ray systems, such as "fan beam" and "flying spot" systems which sweep the area of the imaged object with a narrowed x-ray beam, permit exposure to be varied for different parts or within different zones of the image. Implementation of automatic exposure control in such systems requires an x-ray detection system that can provide exposure readings for individual zones over the entire image area.